This invention relates to an apparatus and process for the determination of particle size data such as particle size and density.
Particulate suspensions are common in a variety of industries including ceramics, metals, foods, medical research, pharmaceuticals, pesticides, cosmetics, and paints and pigments. The two major categories of control parameters for the processing of particle/fluid suspensions are particle physics and interparticle chemistry Funk, J. E., and D. R. Dinger, Predictive Process Control of Crowded Particulate Suspensions Applied to Ceramic Manufacturing, Kluwer, Boston (1994). One of the most important particle physics parameters to measure and control is the particle size distribution (PSD) of the powder. In suspensions and in powder processing in general, PSD affects many process and product properties including particle packing densities, the nature and number of interparticle contacts within a compact, the interparticle porosity and pore size distribution in a compact, suspension rheology and viscosity, drying and firing behavior, and other related properties.
Control of the PSD of powders in a body is effectively complete when the powders have been poured into the mixing device. Frequently, fluctuating body properties caused by variations in powder PSDs are adjusted using additive chemicals, that is, body property variations caused by one parameter (particle physics) are corrected by adjusting the other parameter (interparticle chemistry). Adjusting one control parameter to overcome variations caused by another is not advisable but common. To successfully process any particulate system, it is important that particle physics variations be controlled by adjustments to particle physics. Interparticle chemistry imbalances can then be controlled by adjustments to chemistry and the whole suspension can function as designed. To do this, one must be able to quickly and accurately measure the complete PSD of all constituent powders used in a batch formulation.
A major problem with all of the common particle size analysis techniques available today, including sedimentation, Coulter principle, dynamic light scattering, and turbidity measurement is the requirement of extremely dilute suspensions. These techniques cannot be used at normal suspension densities and require careful sampling followed by dilution and further sampling to achieve the small, highly dilute samples required for the analyses. In addition, these techniques often require calibration on the process stream they are intended to monitor.
Representative examples of prior art particle analysis systems include U.S. Pat. Nos. 5,818,583; 5,940,177; 5,861,951; 5,835,211; 5,831,730; 5,751,423; 5,650,847; 5,576,827; 5,455,675; 5,452,602; 5,438,408; 5,369,037; 5,309,215; 5,286,452; 5,245,200; 5,185,641; 5,164,787; 5,164,604; 5,105,093; 5,064,765; 5,007,737; 4,830,494; 4,781,460; 4,779,003; 4,676,641; 4,541,719; 4,473,296; 4,361,403; 4,338,030; and 4,245,909. The U.S. Patents listed above are incorporated herein by reference in their entirety.
To overcome the problems associated with the present techniques for particle sizing, a new optical method based on frequency-domain measurements of photon-migration in scattering suspensions has recently been proposed as set forth in applicant""s recent publications Jiang, H., G. Marquez, and L V. Wang, xe2x80x9cParticle Sizing in Concentrated Suspensions Using Steady-State, Continuous-Wave Photon Migration Techniques,xe2x80x9d Opt. Lett., 23, 394 (1998) and Jiang, H., J. Pierce, 1. Kao, and E. Sevick-Muraca, xe2x80x9cMeasurement of Particle-Size Distribution and Volume Fraction in Concentrated Suspensions with Photon Migration Techniques,xe2x80x9d Appl. Opt., 36, 3310 (1997), and which are both incorporated herein in their entirety by reference. Because the frequency-domain technique depends upon multiply scattered light, it is particularly suitable for non-dilute suspensions and has great potential for on-line process monitoring. In addition, transport scattering and absorption coefficients can be measured separately. Wavelength-dependent absorbances do not distort scattering measurements; hence, the technique does not require calibrations on the suspensions.
However, initial findings make use of a frequency-domain Ti:Sapphire laser system, which does not provide a light beam having a wide range of spectra. Furthermore, the procedures for optical data collection were relatively slow since measurements at multiple wavelengths were needed.
The present invention demonstrates particle sizing in concentrated TiO2 suspensions, using continuous wave-based photon-migration techniques. Compared with the frequency domain techniques, this method allows faster and simpler measurements of optical properties in a scattering medium with the same accuracy as the frequency-domain techniques. Background details on the methodology and underlying theory may be found in Wang, L., and S. L. Jacques, xe2x80x9cUse of a Laser Beam with an Oblique Angle of Incidence to Measure the Reduced Scattering Coefficient of a Turbid Medium.xe2x80x9d Appl. Opt., 34, 2362 (1995); Nichols, M., E. Hull, and T. Foster, xe2x80x9cDesign and Testing of a White-Light, Steady State Diffuse Reflectance Spectrometer for Determination of Optical Properties of Highly Scattering Systems,xe2x80x9d Appl. Opt., 36, 93 (1997); Lin, S. P., L Wang, S. L Jacques, and F. K. Tittel, xe2x80x9cMeasurement of Tissue Optical Properties by the Use of Obliquexe2x80x94Incidence Optical Fiber Reflectometry.xe2x80x9d Appl. Opt., 36, 136 (1997); and Kienle, A., L Ulge, M. S. Patterson, R. Hibst. R. Steiner, and B. C. Wilson, xe2x80x9cSpatially Resolved Absolute Diffuse Reflectance Measurements for Non-Invasive Determination of the Optical Scattering and Absorption Coefficients of Biological Tissue,xe2x80x9d Appl. Opt., 35, 2304 (1996); the above references being incorporated herein in their entirety. The continuous wave-based system is more economical to supply and operate than its frequency-domain counterpart and offers additional advantages as set forth below.
Photon-migration measurements monitor the characteristics of multiply scattered light as it consecutively scatters from particle to particle. Any fluid or solid material in which the scattering effect of reflected or transmitted light may be determined and may be used to advantage with the present process and methodology and include solutions of a wide range of particle concentration along with gaseous samples, tissue evaluations, and other samples of solids, liquids, or gases. Through the spectral measurement of isotropic or reduced scattering coefficient of a sample, such as tissue, the PSD can be recovered using a regularized inverse algorithm. The photon-migration technique is particularly useful in two aspects: it can analyze particles in dense suspensions without dilution since it depends on multiply scattered light and is thus suitable for on-line process monitoring; and, transport scattering and absorption coefficients of the suspensions can be measured separately, so that wavelength-dependent absorbances do not distort scattering measurements. This enables accurate solution of the inverse problem for determining PSD.
In accordance with this invention, it is possible to use continuous wave-based photon-migration measurements for determination of the transport-scattering coefficients. Useful applications of an inverse algorithm for reconstructing PSD in concentrated suspensions is also disclosed. Using a regularized inverse algorithm, the reconstructed PSD""s of TiO2, suspensions with three different concentrations are set forth. The results are in excellent agreement with the size distribution measured with x-ray sedimentation. These reconstructions of PSDs may be obtained both with and without a priori distribution function assumptions.
The continuous wave based photon migration technique depends on a source of radiation capable of being scattered as a result of contact with particles in a suspension or solution. This invention discloses that the visible spectrum in the range of 400 to 800 nm yields desired result. While it is convenient to use the visible spectrum, the invention anticipates that a full range of radiation wavelengths that can be scattered by contact with material in suspension or solution may be employed. Depending upon the nature of the material to be analyzed, desired wavelengths of radiation may be selected.